The characteristic angles of a vehicle suspension are the toe angle and the camber angle.
The total toe angle is the angle formed by the rolling planes of the two wheels of the same axle observing the vehicle from the top (each wheel has its own toe angle which is equal to a fraction of the total toe angle); the toe angle is positive if the two planes cross in front of the concerned axle (with respect to direction of travel) and, on the other hand, the toe angle is negative if the two planes cross behind the concerned axle (with respect to direction of travel).
The chamber angle of a wheel is the angle which is measured between the plane on which the wheel rolls and the middle trajectory perpendicular to the ground; the camber angle is negative when the wheels tend to close towards the vehicle, and therefore two rolling planes of the wheels meet over the road plane and, on the other hand, the camber angle is positive if the two rolling planes of the wheel cross under the road plane.
In a passive suspension of the traditional type, the camber and toe angles vary “passively” (i.e. without any type of external control) as a function of the forces and moments which are applied to the contact patch of the tire (elastic angle variation) and as a function of the vertical juddering of the suspension (kinematic angle variation). In a high performance sport car, the suspension geometry and the rigidity of the spring/shock absorber assembly are dimensioned so that variations of the camber and toe angles are normally very small; consequently, the actual camber and toe angles never significantly differ from the nominal camber and toe angles which are trade-offs to maximize performance, i.e. to optimize the contact patch of the tire on the ground.
Inevitably, the nominal camber and toe angles are trade-offs which always allows to have a good tire-ground contact patch without, however, ever (or hardly ever) having an optimal tire-ground contact patch. The use of active suspensions has been suggested in which, for example, the suspension arms are replaced by corresponding linear actuators which are driven to vary their length in controlled manner, and therefore vary the camber angle and the toe angle of the suspension in controlled manner in order to improve the tire-ground contact patch in all conditions.
An example of a vehicle equipped with active suspension is provided in patent application WO2008078568A1, in which an electronic control unit is connected to a steering sensor which measures the steering angle in real time and is connected to a triaxial gyroscope which is integral with the vehicle chassis and measures linear accelerations with respect to the ground and angular accelerations with respect to the ground in real time and in a three-dimensional reference system; as a function of the measurements received from the steering sensor and the triaxial gyroscope, the electronic control unit cyclically establishes the optimal camber and toe angle values of each suspension, and thus drives the linear actuators of each suspension to confer the optimal camber and toe angles to the suspension.
However, the known active suspension control methods to do not allow to manage the suspension camber and toe angles in optimal manner, particularly when maximum dynamic performance required in a high-performance sports car.
U.S. Pat. No. 4,371,191A1 describes an adjustment system of the suspensions of an automobile which actively modifies the camber angle of each wheel by means of a corresponding hydraulic actuator to optimize tire-road surface contact. U.S. Pat. No. 4,371,191A1 describes directly measuring the three forces acting on each wheel (longitudinal, lateral and vertical) by means of specific sensors, and thus determining the optimal camber angle for each wheel as a function of the three forces (longitudinal, lateral and vertical) acting on the wheel; in particular, the patent discloses that, in each suspension, the optimal camber angle is chosen to maximize the tire-road contact area as a function of the forces acting on the tire, as a function of the positive/negative toe angle of the suspension and as a function of the road surface. According to a further embodiment described in U.S. Pat. No. 4,371,191A1, the toe angles of the front wheels are also varied, in addition to the camber angles, by means of corresponding hydraulic actuators in order to optimize tire-road surface contact.